Optical recursive filter

Abstract

An optical RF bandpass filter employing a noncoherent optical carrier. The filter consists of a section of multimode optical fiber having a length equal to half the modulation wavelength with mirrors deposited at each end of the fiber. The fiber section functions as a resonant cavity for the intensity modulated light which is directed into the fiber and which exits at one end through one of the partially reflecting mirrors.

What is claimed is: 
     
       1. An optical recursive RF bandpass filter for extracting light varying in intensity at a predetermined intensity modulation frequency from light varying in intensity over a band of intensity modulation frequencies, comprising: a segment of multimode optical fiber;   a first mirror on one end of said fiber;   a second mirror on the other end of said fiber; and   means for directing light varying in intensity over a band of intensity modulation frequencies at a substantially constant optical wavelength into said fiber;   wherein said fiber has a length equal to half the wavelength of a predetermined intensity modulation frequency in said band of intensity modulation frequencies in said fiber, said fiber with said first and second mirrors on opposite ends thereof defining a resonant cavity which permits resonance of light at said predetermined intensity modulation frequency, a portion of the resonating light within said resonant cavity exiting through one of said first and second mirrors as filtered output light varying in intensity at said predetermined intensity modulation frequency with said filtered output light being at said optical wavelength.   
     
     
       2. The filter recited in claim 1 wherein said first and second mirrors are formed of a dielectric material deposited on the respective fiber ends. 
     
     
       3. The filter recited in claim 1 wherein said light directing means comprises a directional coupler. 
     
     
       4. The filter recited in claim 3 wherein said directional coupler comprises a light injection fiber coupled to said multimode optical fiber. 
     
     
       5. The filter recited in claim 1 wherein said light directing means comprises an axial hole through one of said first and second mirrors and an injecting fiber axially aligned with and closely adjacent said hole. 
     
     
       6. The filter recited in claim 5 wherein said injecting fiber is a single mode fiber. 
     
     
       7. The filter recited in claim 1 wherein said optical fiber comprises a core and cladding of different refractive index. 
     
     
       8. The filter recited in claim 1 or 2 wherein the reflectance of the output mirror is less than the reflectance of the other mirror. 
     
     
       9. The filter recited in claim 1 or 2 wherein the reflectances of said mirrors are equal. 
     
     
       10. The filter recited in claim 1 wherein the length L of said filter is given by the relationship   L=λ/2=c/(2nf.sub.c)     where   λ is the wavelength of said predetermined intensity modulation frequency,   c is the speed of light,   n is the effective refractive index of said fiber, and   f c  is the center modulation frequency.   
     
     
       11. The filter recited in claim 1 wherein the light directed into said resonant cavity exists as noncoherent light therein which resonates at said predetermined intensity modulation frequency. 
     
     
       12. The filter recited in claim 11 wherein the light directed into said resonant cavity is noncoherent. 
     
     
       13. The filter recited in claim 11 wherein the light directed into said resonant cavity is coherent. 
     
     
       14. The filter recited in claim 13 wherein the length of said fiber has a value other than one half the wavelength of the light directed into said fiber. 
     
     
       15. The filter of claim 11 wherein said filtered output light has a predetermined 3 dB intensity modulation frequency bandwidth about said predetermined intensity modulation frequency. 
     
     
       16. The filter recited in claim 1 wherein the length of said fiber has a value other than one half the wavelength of the light directed into said fiber. 
     
     
       17. A method for filtering light varying in intensity over a band of intensity modulation frequencies to provide light varying in intensity at a predetermined intensity modulation frequency, comprising the steps of: providing a multimode optical fiber having a pair of mirrors each on a respective end of said fiber, said fiber having a length equal to half the wavelength of a predetermined intensity modulation frequency within a band of intensity modulation frequencies in said fiber, said fiber with said pair of mirrors defining a resonant cavity which permits resonance of light at said predetermined intensity modulation frequency,   directing light varying in intensity over a band of intensity modulation frequencies at a substantially constant optical wavelength into said fiber, said light directed into said resonant cavity resonating at said predetermined intensity modulation frequency; and   extracting a portion of the resonating light in said resonant cavity through one of said mirrors as filtered output light varying in intensity at said predetermined intensity modulation frequency with said filtered output light being at said optical wavelength.   
     
     
       18. The method recited in claim 17 wherein the light resonating in said resonant cavity exists as noncoherent light therein which resonates at said predetermined intensity modulation frequency. 
     
     
       19. The method recited in claim 18 wherein the light directed into said resonant cavity is noncoherent. 
     
     
       20. The method recited in claim 18 wherein the light directed into said resonant cavity is coherent. 
     
     
       21. The method recited in claim 18 wherein the length of said fiber has a value other than one half the wavelength of said light directed into the fiber.